OBJECTIVE:
To review the main aspects of fetal immune development focusing on the host
defenses of extremely preterm infants against bacterial pathogens, and describing
the possibilities of immunotherapeutic intervention for the prevention of neonatal
nosocomial sepsis.SOURCES OF DATA: Electronic search of MEDLINE database for articles published
in the last 15 years. Those with relevant information regarding the target issue
were selected.SUMMARY OF THE FINDINGS: Immunity of extremely preterm infants is deficient
due to skin fragility, insufficient complement system components, decreased
bone marrow neutrophil storage pool, and lower chemotaxis, adherence, deformability,
and neutrophil enzyme activities. Further limitations are found at NK cell-mediated
cytotoxicity, T cell proliferation and cytokine production, B and T cell cooperation,
and antibody synthesis by B lymphocytes. No definitive benefits of interventions
for enhancing the immune function, such as the use of intravenous immunoglobulin
or myeloid colony-stimulating factors, have been demonstrated. CONCLUSION: As a consequence of the immaturity of several immune components,
extremely preterm infants are highly susceptible to nosocomial infections. The
very limited possibilities for intervention in this system require the control
of extrinsic factors for the prevention of nosocomial sepsis in these infants.

For decades, improvements
in intensive care have allowed for the longer-term survival of extremely preterm
newborn infants (those weighing less than 1,000 g at birth). These improvements
have been made not only in industrialized countries, but also in Brazil. Twenty
years ago, 50% of infants weighing less than 1,500 g at birth, treated at Hospital
das Clínicas of the School of Medicine of Ribeirão Preto (HCFMRP-USP)
would not survive, compared to19% at the present time. This improved survival
rate has been associated with a higher incidence of nosocomial infection in
neonatal intensive care units. The risk of sepsis is inversely proportional
to gestational age. The prevalence of sepsis and meningitis has been estimated
in different populations of newborns, ranging from 1 to 5/1,000 live births.
However, for preterm infants, this prevalence is expected to be as much as 1/230
newborns, and sepsis is quite common in this group of patients.1

As examples of
the high frequency of nosocomial infection in extremely preterm newborn infants,
recent data from U.S. neonatal surveys2 indicate an overall sepsis
incidence of 21%, with a significantly higher frequency, the younger the gestational
age ( Table 1). In Brazil, according to an observational
study carried out in 2001 in seven neonatal intensive care units, antibiotic
therapy had to be administered for over five days in 290 of 514 (56.6%) newborns
with gestational age less than 34 weeks, and in 81 of them (27.9%) the microbiological
blood culture yielded positive results.3 Epidemiological surveillance
data on nosocomial infections in the neonatal intensive care unit of HCFMRP-USP,
in the year 2001, indicated an infection incidence rate of 57.3 and 43.9 per
100 infants at risk and those weighing < 1,000.g at birth and between
1,001 and 1,500 g, respectively.4 In this unit, the incidence densities
of nosocomial infection observed in this period were 34.6 and 42.7 per 1,000
patients-day in preterm infants weighing < 1,000 g and between 1,001
and 1,500 g, respectively.4 Also, among extremely preterm infants,
higher nosocomial infection incidence rates were observed at another Brazilian
NICU.5 Usually, mortality from neonatal sepsis ranges from 15 to
60%, depending on gestational age, on the pathogen detected, on the age at which
the disease occurs in the neonatal period and on the quality of intensive care.1
The mortality rate for septic newborns weighing less than 1,500 g was 18%, significantly
higher than that for infants who did not have sepsis (7%) in the U.S. neonatal
survey.2 In the NICU of HCFMRP-USP, 17.6% of 324 newborns under surveillance
in the year 2001 died; death was significantly more frequent among infants diagnosed
with nosocomial infection than among those without this diagnosis (21.6 versus
12.8%). Infection was the cause of death in most infants (75.7%) who had one
or more infectious episodes, mainly those with birthweight < 1,000 g.4

This dataset shows
that it is important to understand the factors associated with nosocomial sepsis
in extremely preterm newborns. Several factors predispose these infants to nosocomial
sepsis:

1. Extrinsic
factors often expose preterm newborns to infection during their hospital stay.
Among these factors are the length of hospital stay, use of invasive procedures
(arterial and venous catheters, parenteral nutrition, tracheal cannulas, gastric
or gastroduodenal tube, ventriculoperitoneal shunts, chest drains, etc.),
characteristics of the exposure to the hospital environment and to the hospital
staff, i.e., the nurse/patient ratio, physical area, personnel training, hygiene
techniques, nosocomial infection control techniques and the pattern of antimicrobial
use at the NICU. Overcrowding, insufficient number of caregivers and the antibiotic
selective pressure increase the chances of acquisition of potentially virulent
pathogens.6

2. Intrinsic
factors are concerned with the immature development of the immune system and
of protective functions of the skin, mucous membranes and gastrointestinal
tract. Birthweight is probably the substitute marker for these intrinsic factors.
However, disease severity varies among infants who have the same birthweight
and is an important risk factor for morbidity and mortality.

In general, some
bacterial infections are not well limited by newborns, and therefore there is
a tendency towards systemic dissemination. Some functions of the immune system
are immature, whereas other aspects are functional at birth, even in extremely
preterm newborn infants. In the present article, we will review the major immunological
peculiarities of extremely preterm newborns, in order to assess the rationale
behind their susceptibility to infection.

General aspects
of the immune system

The main function
of the immune system is to protect the host against microorganisms.7

The first line
of defense against invaders is the physical barrier made up of the keratinized
skin, mucous membranes that line the respiratory and gastrointestinal tracts
and chemical barriers containing a series of enzymes and other substances that
have a direct antimicrobial action or inhibit microbial adherence to body surfaces.

Any invader that
crosses this first line of defense will be attacked by the components of the
innate immune system and, afterwards, by the specific immune system.

Innate immunity
involves humoral elements such as complement system proteins, acute phase proteins,
cytokines and cellular elements such as monocytes, macrophages, granulocytes,
dendritic cells and natural killer (NK) lymphocytes. This system has a limited
capacity to distinguish between microorganisms and often has a similar response
to different microorganisms.

The components
of specific immunity are the lymphocytes and their products (e.g.: antibodies).
In opposition to innate immunity, this system responds specifically to each
microorganism, has a memory and is able to respond more vigorously to repeated
exposures to the same antigen.

Innate and specific
immunities work conjointly. Innate immunity protects against microorganisms
and also plays a key role in the induction of specific response, and this response
enhances the action of protective mechanisms of innate immunity.8

With regard to
newborns, another immunological component that acts while the maturation of
their system is underway is the immunity passively acquired from the mother,
by means of IgG antibodies and human milk.

Development
of the immune system

The immune system
is made up of cells derived from the precursor cells of the hematopoietic system
whose major source is the yolk sac up to the third week of life, followed by
fetal liver at eight weeks and finally by the bone marrow after the fifth month
of gestation. During intrauterine life, these cells are submitted to the effect
of specialized microenvironments, such as bone marrow and thymus, responding
to signs of stimulation, proliferating and differentiating in order to form
the innate and specific immune system. This is a complex and detailed process.
Table 2 shows some moments of differentiation
of this system during intrauterine life. The functions of the different components
in preterm newborns will be discussed next.

Mechanical barriers
and immunity of mucous membranes

The skin of the
preterm newborn, especially of the extremely preterm infant, is immature and
inefficient as epidermal barrier. In normal fetal development, the stratum corneum,
responsible for the epidermal barrier function, only becomes mature around the
32nd or 34th week of gestation.9 Even though maturation accelerates
after birth up to the second postnatal week, the immature skin of extremely
low birthweight infants is more susceptible to ruptures, which facilitates the
penetration of germs.

There is little
information about the maturation of the fetal immune system regarding the mucosal
compartment. The main component of this compartment is the secretory immunoglobulin
A (SIgA). Seidel et al.10 found similar SIgA levels in the saliva
of full-term and preterm infants (mean gestational age of 30 weeks) in the first
nine months of life. Nevertheless, other authors reported lower SIgA levels
in preterm infants at the age of 3 to 8 postnatal months.11

Another important
barrier to infection is that of the gastrointestinal tract, which is impaired
in preterm infants, since the protective gastric acidity is adversely modified
by the virtually continuous feeding of newborns, which increases pH, or by the
occasional use of H2 blockers.12 Assessment of the gastrointestinal
immunologic component shows that only small amounts of the secretory component
are detected up to the 29th week of gestation; also, the immunocytes that produce
IgA, IgM or IgG are scarce up to the first postnatal week. However, after the
second postnatal week of infants born between the 24th and 32nd gestational
week, there is intense epithelial expression of histocompatibility antigens
and development of SIgA, suggesting that this defense system is modulated in
response to environmental factors after birth.13

Innate immunity

Complement
system

This system consists
of approximately 20 proteins produced mainly by the liver and found abundantly
in blood and tissues, requiring sequential activation. There are three forms
of activation of the complement system: alternative, lecithin-dependent, and
classic. The first two are activated in a nonspecific way by contact with certain
components on the surface of microorganisms, regardless of the presence of specific
antibodies. Consequently, a series of substances is formed (C3a, C3b, C5a, among
others) that release inflammatory mediators, stimulate chemotaxis and phagocytosis
and, when the activation is complete, cause microbial lysis through the components
of the membrane attack complex (C5b-C9).7 The classic pathway is
initiated by the binding of circulating antibodies to the first protein (C1)
of the complement system.8

The elements of
the complement system are detected in the fetus early on during gestation, but
the levels of these proteins remain low up to the last trimester. There is no
transplacental transfer of complement system elements, but the fetus and the
newborn can synthesize them and this ability increases with gestational age.14
At the end of gestation, there is an increase in C3 of up to 60-80% of
the value observed in adults, but the final components of the complement system
may reach only10% of maternal levels. Despite this quantitative deficiency,
the functional capacity in full-term newborns, determined by the total hemolytic
complement activity (CH50), is similar to that observed in adults. The components
of the alternative pathway in newborns reach 35 to 70% of adult levels. Preterm
newborns have lower complement and lytic activities levels at birth, and the
levels of C3, C4 and CH50 increase with gestational age.15 Low levels
of early components in preterm newborns cause deficiency of activation products
that are crucial to chemotaxis and opsonization.

Phagocytes

Neutrophils, monocytes
and macrophages are able to phagocytose microorganisms and destroy them intracellularly
by the action of several toxic substances, including superoxide anions, hydroxyl
radicals, nitric oxide, cationic protein, hypochlorous acid and lysozyme. Neutrophils
and the mononuclear phagocyte system cells originate in the bone marrow from
granulocyte and monocyte-macrophage cells, the colony-forming unit granulocyte-monocyte
(CFU-GM) pool. Glycoprotein hormones, known as colony stimulating factors (G-CSF/
GM-CSF), induce the proliferation, maturation and differentiation into neutrophils
and monocytes.16

Macrophages and
dendritic cells are the main soluble antigen-presenting cells that induce the
proliferation of T lymphocytes by the secretion of cytokines that provide signs
for the activation. To do that, it is necessary to have the expression of major
histocompatibility (MHC) antigens of classes I and II on the cell surface. Around
the 12th week of gestation, the expression of these molecules is evident in
several fetal tissues, including macrophages and dendritic cells in a similar
way to that of adults.17 However, some data indicate deficiencies
in the processing and presentation of antigens, and failure in the regulation
of the expression of MHC II on the cell surface, thus minimizing this function
in newborns.18 With regard to neonatal monocytes, deficiencies such
as low production of cytokines have also been observed.19

Neutrophils (polymorphonuclear
leukocytes) are able to respond and migrate quickly and in large numbers to
the lesion or infection site. Since these cells are essential components of
innate immunity, they have been extensively studied in newborns regarding body
components and functions of chemotaxis, deformability, adherence, phagocytosis
and bacterial lysis.

The bone marrow
neutrophil storage pool in newborns is smaller than that of adults and may become
totally depleted in the presence of sepsis. In infants born with less than 32
weeks of gestation this storage is equivalent to nearly 20% of the pool of full-term
newborns and adults.20 In these extremely preterm newborns, the occurrence
of neutropenia during the course of an infection is often due to the depletion
of bone marrow reserves.21

Neutrophils remain
inactive in the bloodstream and circulate at a high speed.8 Neutrophil
activation occurs in the tissues, at sites of infection, but before they get
there they need to come into contact with the endothelium, adhere, stop, deform
and cross endothelial cells. The transient contact of neutrophils with the endothelium,
recognized in dynamic videomicroscopic images such as "cell rolling,"
depends on the association and dissociation of receptors and their ligands in
endothelial cells and at the apices of villous projections of neutrophils. The
main molecules involved in these interactions are the placental selectin (P-selectin),
endothelial selectin (E-selectin) and leukocyte selectin (L-selectin).16
P-selectin, expressed in platelets and endothelial cells, is subtly expressed
by the fetal endothelium before the 27th week of gestation and its expression
is basically normal in full-term infants and low in preterm newborns.22
L-selectin is basically expressed by leukocytes, with low levels in the cord
blood of newborns, especially in preterm newborns.23,24 In the first
month of life, the expression of L-selectin is low in the cells of newborn infants
comparatively to the cells of adults.25 The firm adhesion of neutrophils
to the endothelium is mediated by beta-2-integrins. The most important adhesin
in this group is the complement receptor 3 (CR3 or CD11b/CD18). CR3 is expressed
at low concentrations on the surface of inactive neutrophils. CR3 recognizes
the members of the superfamily of intercellular adhesion molecule-1 (ICAM-1)
and ICAM-2 to which they bind, resulting in the firm adhesion of the neutrophil
to the endothelium. Inactive neutrophils in newborns have a CR3 expression similar
to the neutrophil expression in adults.25 The stimulation and activation
by inflammation increase in vivo and in vitro CR3 complexes, but
this increase is significantly lower for neonatal neutrophils.25,26
The total CR3 concentration in neonatal neutrophils increases with gestational
age up to 60% of the value observed in adults.26

After adhering
to the vascular endothelium, neutrophils have to deform in order to cross the
junctions between endothelial cells. To some extent, deformability depends on
the formation of actin filaments from the polymerization of monomeric G-actin.
Neutrophils in newborns form fewer actin filaments after stimulation with chemotactic
factors than neutrophils in adults.27 The neutrophil membrane is
another determinant factor for deformability. The increased fluidity of neutrophil
membrane in newborns results in reduced deformability.28

Outside the blood
vessels, neutrophils move towards the site of infection, guided by chemotactic
molecules such as the C5a fragment of the complement system, interleukin-8,
leukotriene B4, platelet activating factor, and fragments of bacterial proteins
known as formyl-methionine peptides (f-met).8

At the site of
microbial invasion, a contact is established between the pathogen and phagocyte
through opsonin binding (immunoglobulin G (IgG) and C3b/iC3b) to the pathogen
with specific receptors on the cell surface. The interaction between opsonins
and receptors along with the activation of contractile elements produces pseudopods
that engulf the pathogen forming a phagocytic vacuole that is internalized and
suffers the sequential action of antimicrobial proteins and hydrolytic enzymes
released by neutrophil granules.16 The bactericidal permeability
increasing protein (BPI) that neutralizes the lipopolysaccharide endotoxin (LPS)
and is cytotoxic to gram-negative bacteria has a concentration three to four
times lower in full-term newborns compared to adults.29 The amount
of BPI found in the neutrophils of preterm newborns is even lower than that
of full-term newborns.30

During phagocytosis,
there is a sudden increase in the cellular metabolism of oxygen (respiratory
burst) with the production of oxygen metabolites including superoxide anion
[O2-], hydrogen peroxide [H2O2] and the hydroxyl
radical [HO], which have bactericidal activity. The interaction of oxygen metabolites
with excitable substrates in the cell also leads to a chemiluminescence burst.
It is controversial whether the increase in respiratory activity of neutrophils
in newborns is identical to that of adults and whether that of preterm infants
is lower than the one observed in full-term infants.31,32 Recently,
Björkvist et al.33 have shown that neutrophils in preterm infants
have a lower capacity to increase their respiratory activity after stimulation
with coagulase-negative staphylococci than do neutrophils in full-term infants.

NK cells

These cells are
large and granular lymphocytes that often express the IgG receptor (CD16) and
the CD56 marker. Their ability to produce cytotoxicity enables them to lyse
infected cells or antibody-sensitized cells in a direct fashion. These cells
act mainly against tumor cells and virus-infected cells. However, they are also
able to lyse bacteria, parasites, fungi and some normal cells in the absence
of previous sensitization. Comparatively to adults, newborn infants have similar
or higher amounts of NK cells in peripheral blood. Nevertheless, there are phenotypic
and functional differences, for instance, the cytotoxic activity is lower in
newborns.34 Few studies have been carried out with preterm infants
and have shown that these infants have lower NK activity than full-term newborns,
and also a smaller amount of cells.35 In these infants, there is
an increase in the amount of NK cells and in the lytic capacity of these cells
after birth, which may result in improved cytokine production35 in
response to the stimulation from the extrauterine environment.

Specific
immunity

T lymphocytes

They are specialized
cells of the immune system that, after being stimulated by antigen-presenting
cells and being activated, respond to new antigens by producing or expressing
cytokines in their cell membrane that amplify or regulate the several aspects
of immune response.7 In addition to effector functions, these cytokines
take part in key effects for the proliferation of NK cells, monocytes, B lymphocytes
and for the proliferation of T lymphocytes as well. T lymphocytes are subdivided
into various subtypes according to their surface markers and to their cytokine
production.8 The two largest subpopulations of T lymphocytes that
are important to specific immunity are the CD4+ or T helper lymphocytes and
the CD8+ or cytotoxic lymphocytes. The function of CD4+ cells is to activate
macrophages and encourage B lymphocytes to produce antibodies. The population
of CD8+ cells plus that of NK cells mediates most cytotoxic activities against
virus-infected cells or tumor cells. At the initial stage of an infection, the
infected cells are nonspecifically lysed by NK cells, which controls the infection,
but eradication occurs through the clonal expansion of antigen-specific cytolytic
cells (CD8+).8

Neonatal T lymphocytes
exhibit deficiencies such as low proliferative response, lower interleukin-2
(IL-2) production, decrease in the cytolytic activity, and abnormal production
of cytokines.18 Thus, the responses of the fetus and newborn infant
to specific T-dependent antigens, including CD8+-mediated cytotoxicity 36
and the production of CD4+-dependent antibodies37 is reduced or delayed,
in comparison to other individuals.

With regard to
cell-mediated immunity in extremely preterm infants, some peculiarities distinguish
them from adults and full-term infants. The major differences are the decrease
in the in vitro proliferation after phytohemagglutinin stimulus and the
smaller number of CD3 and CD8 cells.38 However, comparatively to
adults and full-term infants, the response to some microbial pathogens (H.
influenzae, S. epidermidis) is enhanced on the first day of life, but this
response decreases two weeks after birth.39

B lymphocytes

The major function
of the population of B cells is the production of several spectra of immunoglobulin
molecules that constitute the humoral component of the specific immune response.
From precursor cells, plasma cells are the most differentiated ones for the
production of antibodies. In adults, these cells are able to synthesize thousands
of molecules per second. These cells are activated by the interaction with T
lymphocyte and the antigen. In response to the antigen challenge, the production
of IgM is the primary response.7

The infected fetus
and newborn infant are able to produce IgM antibodies in response to bacterial
antigens, but at lower levels than that of adults. However, the synthesis of
IgG and IgA is limited.14 On top of that, these antibodies do not
respond to certain antigens, especially bacterial polysaccharides and have limited
capacity to develop memory. This limited production of antibodies may be due
to the development of B cells and to the absence of signs of stimulation.14

Passively
acquired immunity

The transplacental
transfer of maternal immunoglobulin to the fetus is a specific adaptation that,
to some extent, minimizes these deficiencies in the production of antibodies.
The transfer of IgG is limited to IgG isotopes. The IgG Fc fraction has been
regarded as the main fraction implicated in the transplacental transfer of this
immunoglobulin. The presence of receptors for the Fc region of IgG has been
observed in human placenta in the first 12 weeks of gestation. The syncytiotrophoblast
and Hofbauer cells are the main cells that contain Fc receptors.40
All of the four IgG subclasses (IgG1, IgG2, IgG3 and IgG4) cross the placenta.
The fetus receives the antibodies against the antigens to which the mother was
exposed due to colonization, infection or vaccination. If the mother has small
concentrations of antibodies or if the protective antibodies are not of the
IgG isotype, as occurs with IgM antibodies against gram-negative bacteria, such
as E. coli and Salmonella sp, the fetus will not receive them.14

Studies on the
pattern of transfer of different types of specific IgG antibodies show that
there are peculiarities in the transfer of these antibodies. The placental transfer
is more efficient for IgG1 and IgG3 and less efficient for IgG2.41
Therefore, the transfer of antibodies against viral proteins of the IgG1 subclass
(poliovirus, measles, rubella, mumps) and antitoxins (tetanus, diphtheria, erythrogenic)
occurs more easily. However, antibodies against encapsulated bacteria (Haemophylus
influenzae, Bordetella pertussis, Neisseria meningitidis, Streptococcus pneumoniae),
in which IgG2 prevails, are transferred less efficiently, i.e., only 50 to 60%
of those detected in the mother.42

The active transport
of immunoglobulin G via the placenta begins early on and increases proportionally,
and in this case, gestational age influences the total IgG levels in umbilical
cord blood, with a linear correlation between the concentrations of fetal IgG
and gestational age. Around the 32nd week of gestation, the detectable levels
in the newborn are of approximately 400 mg/ml, but levels greater than 1,000
ml/dl can be achieved in full-term infants, being sometimes even higher than
maternal levels.14 Preterm infants, especially extremely preterm
ones, may not receive protective antibodies, as most of these antibodies are
transferred to the fetus after the 34th week of gestation.43 Thus,
as also occurs with full-term infants, this deficiency is even more pronounced
for IgG2 antibodies.

Table
3 summarizes the main characteristics of the premature newborns immune system
components previously mentioned.

Human milk and
the colonization by the intestinal microbiota obtained from the mother are other
essential adaptations for the passive protection of newborn infants. Human milk
contains several protective elements. In addition to protecting infants in a
passive way, human milk stimulates the immune system of newborns by way of anti-idiotypic
antibodies and through the uptake of lymphocytes, cytokines and other elements.44

Among other protective
factors, SIgA, the isotype predominantly found in human milk, provides protection
against all the microorganisms that the mother may or may not have in her digestive
tract, preventing them from adhering to the mucous membranes. Lactoferrin, the
main protein component of mature milk, is relatively resistant to enzymatic
digestion and has antimicrobial, immunostimulatory and antiphlogistic effects,
by decreasing the synthesis of proinflammatory cytokines such as IL-6, IL-8
and TNF.45 The oligosaccharide fraction of milk contains analogues
of several receptors for microorganisms in the mucous epithelium. This way,
the presence of human milk during neonatal colonization and the subsequent expansion
of the intestinal microbiota is an essential factor not only for the prevention
of infections, but also for the induction of the immune system maturation.44
Extremely preterm infants, due to different factors, often are not allowed to
be fed milk from their own mothers or from a milk bank, which also contains
most of the immune properties.46 Consequently, intestinal colonization
occurs through an unbalanced microbiota made up of bacteria found in intensive
care units, many times virulent bacteria submitted to the antimicrobial selection
pressure, which favors the invasion of these microorganisms.

Proposals for
improvement of the immunity of extremely preterm infants and prevention of nosocomial
infection

As previously
described, preterm infants have several factors that favor bacterial invasion
and the occurrence of neonatal sepsis. The immature mechanical barriers, the
limited functions of neutrophils, the low plasma concentration of specific antibodies,
the low activity of complement system proteins and the poor cooperation between
T and B lymphocytes predispose these infants to bacterial invasion.

Knowledge about
several characteristics relative to the development of the fetal immune system
and about the deficiencies of defense mechanisms for the protection against
neonatal pathogens opened the path for potential interventions aimed at enhancing
defenses and preventing or treating nosocomial infection.

We will review
the main proposals regarding immunomodulation for the prevention of infection
in extremely preterm infants.

Topical skin
emollients

Prevention of
skin barrier breakdown and of bacterial penetration through the skin using topical
emollients has been proposed. However, although their use in the first 15 days
improves the condition of the skin and prevents loss of water through the skin,
there is no evidence of protection against bacterial invasion. Conversely, data
obtained from a study including 1,191 newborns weighing between 501 and 1,000
g at birth and gestational age < 32 weeks assigned to receive generalized
application of emollients twice a day or selective application on the site of
the skin lesion, instead of having a protective effect against infection, revealed
a higher risk of bacterial nosocomial sepsis.47 Similarly, a recent
meta-analysis of the available studies showed that the prophylactic application
of topical emollients in extremely preterm infants increased the risk of any
nosocomial infection and, mainly, of infections caused by coagulase-negative
staphylococci.48 Consequently, the therapeutic use of emollients
has been proposed only for the sites with skin lesions. However, no studies
have validated their use.

Intravenous
immunoglobulin

The use of intravenous
immunoglobulin (IVIg) aims to allow IgG to bind to receptors on the cell surface,
promote opsonic and antibody-dependent cytotoxic activities, activate the complement
and improve neutrophil chemotaxis. After noting that the serum immunoglobulin
level is low in extremely preterm infants, the administration of intravenous
immunoglobulin from a pool of adult donors would seem logical for the prevention
of nosocomial sepsis. Although it has been shown that its administration is
safe, its efficacy remains arguable.

The results of
several studies assessing the efficacy of the prophylaxis against sepsis in
preterm infants through intravenous administration are controversial; some of
them suggest benefits while others do not. Baker et al.49, by studying
558 newborns weighing between 500 and 1,250 g, found a lower risk of nosocomial
infection and length of hospital stay in those infants who received IVIg (500
mg/kg/day) than among those treated with placebo. Fanaroff et al.50
carried out a controlled clinical trial that included 2,416 newborns weighing
less than 1,500 g who received intravenous doses of 700-900 mg/kg of IVIg or
placebo every 14 days until they achieved 1,800 g, but no reduction in the incidence
of nosocomial infection, mortality or other outcomes was observed. Usually,
the discrepant results are attributed to the variable titers of specific immunoglobulins
against the causative agent of nosocomial sepsis.

Recently, Ohlsson
et al.51 have conducted a meta-analysis of the data about 19 placebo-controlled
or noninterventional studies with the aim of gathering information on the largest
possible number of infants who received intravenous immunoglobulin as prophylaxis
during eight or more days, including more than 5,000 newborns weighing less
than 2,500 g. A 3% reduction in the incidence of sepsis and a 4% reduction in
the occurrence of one or more episodes of any severe infection were obtained.
These results showed that it would be necessary to treat 33 and 25 infants in
order to prevent one case of sepsis and one case of any other severe infection,
respectively. However, no decrease in mortality was observed, nor in the occurrence
of necrotizing enterocolitis, bronchopulmonary dysplasia, intraventricular hemorrhage
or in the length of hospital stay.

Currently, the
use of formulations containing hyperimmune IgG antibodies against specific antibodies
and/or IgM antibodies has also been considered. However, clinical trials are
not available yet.52 It should be underscored that there are no available
data justifying the routine administration of intravenous immunoglobulin for
the prevention of nosocomial sepsis. This use is justifiable in units with a
high incidence of nosocomial infection and in which the incidence remains high
even after control measures against infection are strengthened. Nevertheless,
the prophylactic use should be based upon a careful assessment of costs and
clinical benefits.

Myeloid colony-stimulating
factors

These are hematopoietic
growth factors that promote the proliferation, differentiation, maturation,
survival and activation of neutrophils and macrophages. Their cord blood levels
are correlated with gestational age21 and the production by mononuclear
cells in fetuses and preterm infants is significantly lower than in full-term
newborns.53 Given the potential capacity to improve the phagocytosis
of bacteria and fungi and the frequent occurrence of neutropenia and impaired
neutrophil function, the application of these recombinant human factors was
analyzed in terms of prophylaxis and treatment of neonatal sepsis. Two factors
were evaluated in newborn infants: granulocyte colony-stimulating factor (G-CSF)
and granulocyte macrophage colony-stimulating factor (GM-CSF).

It has been well
documented that both factors increase the number of circulating neutrophils,
increase the bone marrow neutrophil storage pool and increase the expression
of neutrophil C3bi receptors, in the absence of short or long-term adverse effects.16
However, the administration of these factors did not remarkably reduce the incidence
of nosocomial sepsis in most extremely preterm infants.54

Carr et al.55
performed a meta-analysis of three controlled studies that aimed to improve
immunity and reduce the incidence of nosocomial sepsis and mortality from infection.
Altogether, these studies included 359 newborn infants with less than 32 weeks
of gestational age or weighing less than 1,000 g, either with or without neutropenia,
and who received GM-CSF immediately after birth. Due to the variability of studies,
it was not possible to verify whether the administration of GM-CSF reduced the
incidence of nosocomial sepsis, but its administration did not cause any decrease
in mortality. Therefore, up to the present time, there are not sufficient data
to recommend the prophylactic routine administration of GM-CSF for the prevention
of nosocomial sepsis.

These stimulators
provide protection against infection when given to preterm infants with less
than 32 weeks and who are neutropenic (< 1,700/mm3) or who are
at risk for neutropenia in the postnatal period. However, only one controlled
study has been conducted with this group of infants,56 wherein the
incidence of systemic infection decreased from 53 to 31%, but without statistical
significance, possibly due to the small number of infants analyzed. A similar
effect was observed in a non-controlled study in which G-CSF was administered
to neutropenic infants.57 By the end of 2004, a large number of patients
will have been recruited for a UK study aimed to verify whether the prophylactic
use of GM-CSF is able to reduce systemic infection or mortality in infants at
high risk for postnatal neutropenia.

Final remarks

Although we know
the immunologic limitations of preterm infants, the alternatives to intervention
have been scarce so far and future studies should explore different combinations
of immunoprophylactic measures, including maternal vaccination58
and newborn infants. Additionally, the measures intended to reduce the virulence
of pathogens that cause nosocomial sepsis are still incipient.59,60
Therefore, a great deal of effort should be made so that preterm infants, especially
extremely preterm ones, are not colonized and invaded by nosocomial pathogens,
since once the infection is established, the consequences will be potentially
deleterious. It is therefore necessary to intervene in factors extrinsic to
extremely preterm infants that are strongly associated with the occurrence of
nosocomial infections. The necessary measures, among which hand hygiene plays
a crucial role,61 are summarized in Table 4
and reviewed in other articles.62,63